Traditionally, tool holders have been designed to accept carbide cutting inserts. Typically, a seat area is formed about the head of the tool holder and a shim is secured about the seat area by a shim screw that threads into an underlying threaded bore. A cutting insert is secured atop the shim by a clamping mechanism.
In order to provide for versatility, tool holders have been designed to accommodate various types of cutting inserts. For example, Kennametal, Inc. of Latrobe, Pa., offers a combination tool holder, known as the "Kenloc" combination tool holder, that is designed to accept different types of cutting inserts having different thicknesses, etc. To accommodate these different inserts it has been necessary to provide different shim thicknesses and different methods of securing the shims to the head of the tool holder. In the Kennametal "Kenloc" tool holder for example, there is provided the option of securing a shim by a threaded lock pin with a concentric flange that enters the shim counterbore, or simply securing a shim by means of a shim retaining screw that enters the shim counterbore and is threaded into the tool holder.
As noted above, the shims are designed to be directly secured to an underlying seat area formed in the tool holder. In order to provide for direct securement to the tool holder these shims are provided with a counterbore opening that is designed to receive a shim securing device such as a shim retaining screw or a concentric type locking pin. It is therefore appreciated that the presence of the shim opening effectively decreases the bearing surface of the shim which in turn provides support for the cutting insert. This decrease in effective shim bearing surface can be significant especially where the shim is designed to be used in a combination tool holder. This is because the area of the shim opening must be large enough to be compatible with the shim securing means and particularly the threaded bore underlying the seating area.
Generally, the bearing surface of the shim has not been critical when the shim is used to support a conventional carbide cutting insert. But in recent years advanced materials such as ceramic, cermet, CBN, and sailon have become popular especially in high-speed machining operations. These inserts, while high in compressive strength, tend to be brittle and relatively low in transverse rupture strength. Thus, the larger the shim opening the less the bearing surface and consequently the potential for rupture or breakage of the cutting insert is increased.
Therefore, there is a need to provide a combination tool holder that is capable of accepting and accommodating various different types of cutting inserts including the so-called advanced material cutting inserts but which is provided with the capability of maximizing shim bearing surface for these hard but brittle inserts.